A control device (200) for a motor drive system (100) including an AC motor (MG1) having a magnet on a rotor, a converter (120) and an inverter (130), generates a boost command value for the converter (120) on the basis of a torque command value of the AC motor (MG1). The control device (200) determines whether weak field control, which increases the current in the direction in which the magnetic force supplied to the AC motor (MG1) from the inverter (130) is diminished, is to be implemented, on the basis of the boost command value and the drive status of the AC motor (MG1). If weak field control is to be implemented, and the absolute value of the torque command value is smaller than a threshold value, the control device (200) will further increase the generated boost command value. As a result of doing so, the level of the weak field current can be reduced, thereby improving the efficiency of the motor drive system (100).